Functional analysis of Bicc1 and novel interacting proteins in renal tubule cell metabolism and polycystic kidneys

Genetic disorders known as ciliopathies develop polycystic kidneys, including autosomal dominant and autosomal recessive polycystic kidney diseases (ADPKD and ARPKD). Several signaling pathways including the cAMP/PKA pathway are implicated in driving cystic growth. However, the mechanisms underlying cysts formation is still elusive. Renal cysts also arise in patients and in mouse models with mutations in the RNA-binding protein Bicaudal-C (Bicc1). Bicc1 binds specific mRNAs such as adenylate cyclase VI (AC6) mRNA for translational repression. Conducting a structure-function analysis, we demonstrated that Bicc1 self-polymerizes through its sterile alpha motif (SAM domain) to form cytoplasmic clusters, which specifically localize and silence bound mRNAs. Altered glucose and lipid metabolism also sustain cystic growth in polycystic kidneys, but the cause of these perturbations is unclear. Here, we found that Bicc1 is essential for normal expression of the gluconeogenic enzymes FBP1 and PEPCK specifically in kidneys, and to maintain euglycemia. In a proteomic interactome screen using a knock-in cell line, we found that TAP-tagged Bicc1 binds the C-Terminal to Lis-Homology domain (CTLH) complex, the mammalian orthologue of the glucose-induced degradation (Gid) complex that triggers degradation of FBP1 in S. cerevisiae. Bicc1 stimulates FBP1 expression in vitro in cultured murine inner medullary collecting duct (mIMCD3) cells, albeit independently of its interaction with the CTLH complex. Instead, Bicc1 and CTLH complex seem to mutually downregulate each other. Bicc1 interactome is also enriched with metabolic related proteins. Here we found that Bicc1 coimmunoprecipitates with rate-limiting metabolic enzymes and contributes to maintain their protein levels, and that deletion of Bicc1 significantly alters the renal metabolome while enhances autophagy. Altogether, these results implicate Bicc1 as an important metabolic regulator in the etiology of polycystic kidneys.